Depending on childrens birthdates, generally 4 year olds would mean junior kindergarten, 5 would be senior kindergarden and 6, grade one and so on. Junior kindergarten is optional. Some parents keep children home for that year if they feel they are not ready, or think they should not start so young. Most send their kids though if they are showing readiness. It definitely was better for adjusting to SK in my opinion.
You can also homeschool.
Schools make it easy to get enrolled and get going-they will be very helpful.